Histological and ultrastructural studies of left ventricular endocardium from 10 patients with endocardial fibroelastosis (EFE) revealed that the average size of elastic fibers in thickened endocardium was much larger in the 4 patients with congenital EFE than in the 6 patients with acquired EFE (secondary to ischemic heart disease in 2 patients, to prosthetic cardiac valves in 3, and to irradiation of the chest in 1). Both components of normal elastic tissue (central, amorphous cores and peripheral microfibrils) were present in endocardial elastic fibers of each patient. Ultrastructural identification of elastic fibers was greatly facilitated by staining with silver tetraphenylporphin sulfonate.